A quick, over-the-shoulder check is not dangerous at all if a driver gives the car in front proper following distance. Too many people follow WAY too closely, especially at highway speeds.

Lastly, safe driving includes both actions. Step 1 is to check your mirrors (first rear, the side views). Step 2 is to quickly check over the shoulder. Step 3 is to make the maneuver.

As a motorcyclist, I have to be vastly more aware and defensive than cagers, because a little fender-bender for drivers means injury and even death for riders. It is hammered into you, at any motorcycle safety course (by Moto Police no less, who spend all day riding motorcycles, every day) that you should always check over your shoulder before making lane changes, turns, etc. This is always done after you have already checked your mirrors.